MMICs are important components for communication systems. This is especially true for wireless systems. Each of the wireless systems requires a transmitting and receiving (T/R) module to send or to receive microwave signals. As shown in FIG. 1, a typical T/R module has a first input matching circuit, a power amplifier (PA) and a first output matching circuit, a switch (SW) and an antenna for transmitting operation. A second input matching circuit, a low noise amplifier (LNA) and a second output matching circuit together with the switch are used for receiving operation. Power supply to PA and LNA, operation conditions and control of PA and LNA are achieved by a bias and control logic (BCL) circuit. The bias and control logic (BCL) circuit is implemented normally by CMOS. PA, LNA and SW are implemented using III-V or SiGe technologies. The PA, LNA and SW are manufactured on a GaAs substrate which is subsequently grinded to a thickness suitable for heat dissipation and for transmission lines. After the grinding, the chips are usually bonded onto a carrier. On the carrier, there are metal patterns to accept bonding wires to complete the electrical connection between the chips. During the die attachment and wire bonding, special precautions are required to maintain the positions of chips and wires in order to maintain integrity of the circuits and to minimize interference. It is also important to keep the unwanted series resistance to be as small as possible so that unwanted heat generation can be minimized. During the die attachment, a layer of electrically and thermally conductive paste is applied between the carrier surface and the chip. Air bubbles or voids are frequently formed in the paste. These bubbles or voids can increase the thermal resistance between the chip and carrier, hence lead to non-uniformity in the heat dissipation of heat from the chip during the operation. Therefore, it would be advantageous to have a method and structure so that the active components and passive components are deposited directly on a substrate to form a module, hence to avoid the need of bonding a semiconductor chip to a carrier in order to form the module.
For modern MMICs in communications, majority of the chip area is for passive components or simply unused. For example, the area occupied by active components is only between 3%-5% in power amplifiers. An exemplary MMIC PA for transmitting as shown in FIG. 2 includes a switch, an input matching network, an amplifier, an output matching network and a bias-control circuit. Areas used by active components are indicated by dotted rectangle. To fabricate active components, epitaxial layers are required as the bases. On the other hand, for passive components such as transmission lines, capacitors and inductors, the presence of epitaxial layers and the associated semiconductor substrate is not only not required but also sometimes detrimental. However, for the epitaxial wafers normally used for the manufacturing of MMICs, the entire surface is covered by uniform epitaxial layers which are deposited by MBE or MOCVD. In order to avoid performance degradation of the passive components, majority of the epitaxial layer which is not occupied by the active components are either removed or passivated by etching or ion implantation.
When monocrystalline GaAs substrates with epitaxial layers are used for MMICs fabrication, thickness of the substrate is usually 700 to 1000 μm, which is required to maintain integrity during handling and processing for MMICs. However, in order to reduce thermal resistance and to obtain standard 50 ohms transmission lines of reasonable line width, thickness of the GaAs substrates has to be reduced. Such thickness reduction is performed after all the active and passive components have been fabricated. Typical thickness values of the GaAs substrate after the reduction is 100 μm, 75 μm or even 50 μm. To reduce the thickness, the back surface of the substrate is first grinded to remove a major part of the substrate material. This is followed by a chemical etching process to increase the smoothness of the grinded back surface. It is thus clear that using GaAs substrate for MMIC fabrication, more than 80% of the starting GaAs substrate material must be removed. The grinding produces particles of GaAs still in compound semiconductor form and often mixed with water, whereas chemical etching produces ions and complexes of gallium and arsenic mixed in water. While ions and complexes of gallium are less harmful and are not considered as severe contaminants, ions and complexes of arsenic are highly toxic and are severe environment contaminants. In addition, the removal of GaAs particles and ions and complexes of gallium and arsenic requires filtering and ion exchanging. These methods are expensive and difficult to achieve using low cost equipment. Furthermore, after the grinding and etching for the back side of the GaAs substrate, via holes must be etched. The surfaces of these via holes must be plated with conductive metal layers in order to achieve grounding and improve thermal dissipation. For efficient transmission of microwaves, the thickness of substrate should be small enough to get 50 ohms microstrip. In addition, the heat generated by the components need to be remove from a heat sink connected to the back side of the substrate. The thickness of the substrate should be small in order to reduce the thermal resistance. After dicing, the chips are mounted on a carrier to complete the module.
In the prior art methods, as shown in FIG. 3, chips with different functions are selected and mounted on a carrier to form a module. The carrier is often a dielectric substrate with metal patterns on top. In addition, the carrier thickness should be small enough for efficient heat dissipation. In order to obtain functional module, the chips must be selected and precisely positioned or aligned on the dielectric carrier with respect to the metal transmission lines created on top of the dielectric carrier. Since the alignment and positioning is achieved by mechanical devices, high precision which is required for high reproducibility is difficult to achieve.
It is clear from the above comments that it is beneficial and desirable to develop MMIC circuits without the need of grinding and etching of the substrate after the fabrication of active and passive components. Furthermore, it is advantageous to develop technology for active devices without the need of compound semiconductors containing toxic elements such as GaAs or InGaAs. The success in the MMIC methods and structures without substrate grinding/etching and using semiconductors without toxic elements for active components will reduce manufacturing time and decrease economic cost and environmental burden. It is also beneficial to have an MMIC structure or method where the requirements for die or chip attachment, alignment and wire bonding are eliminated completely or minimized. This will increase the reproducibility and reduce the manufacturing time for the MMIC circuits and modules.